Method and device for pickling hot-rolled special steel strips

ABSTRACT

The invention relates to a method for pickling hot-rolled special steel strips ( 1 ) having an austenite, ferrite and/or martensite structure. The inventive method is carried out in a pickling line ( 7 ) with pickling sections ( 8 ) containing acids and rinsing sections ( 9 ) containing rinsing water ( 15 ). The aim of the invention is to provide a method that does not require an additional stretch-leveling step for descaling. The costs for pickling are reduced by pickling the hot-rolled special steel strip ( 1 ) in the pickling sections ( 8 ) with hydrochloric acid ( 13 ) and rinsing it in the first rinsing sections ( 9 ) with azotic acid ( 14 ) or an equivalent substitute pickling medium ( 14   b ) and treating it in further rinsing sections ( 9 ) with rinsing water ( 15 ).

CROSS REFERENCE TO RELATED APPLICATION

This application is a national stage of PCT/EP 01/105118 filed 5 May2001 and based upon German national application 100 22 083.5 of 8 May2000 under the International Convention.

FIELD OF THE INVENTION

The invention relates to a method and an apparatus for picklinghigh-grade-steel hot-rolled strip with an austenitic, ferritic and/ormartensitic structure and which is carried out in a pickling line withpickling stages with acid and rinsing stages with rinse water.

BACKGROUND OF THE INVENTION

In the pickling of high-grade-steel hot-rolled strip (DE 195 05 324 C2),the high grade steel strip is passed through an annealing and picklingline. Prior to the pickling a descaling is effected which involves twoprocessing segments: blasting and pickling. In the blasting, the stripsurface is mechanically descaled in that particles impinge upon thestrip surface which remove the scale because of their kinetic energy. Itis known to include in some annealing and pickling lines an additionalstretch leveller so as to improve strip quality with respect to theplanarity of the strip on the one hand and on the other hand to producecracks in the scaling layer which facilitates the later chemicalpickling process. Following the mechanical preliminary descaling steps,a wet chemical pickling is carried out. This pickling process can,depending upon the apparatus configuration, comprise a plurality ofsteps. For purely hot-rolled strip as a rule the pickling involves asulfuric acid prepickling with residual descaling being effected in anacid mixture of nitric acid and hydrofluoric acid.

This known process is disadvantageous because of the use of the blastmethod which increases the roughness of the strip surface and thusreduces strip quality. In addition, the operating cost because of theuse of the acid mixture and especially the hydrofluoric acid is veryhigh.

The method described at the outset is known (EP 0 706 840 A2) for theproduction of high-quality cold-rolled steel strip carried out inlinewith the working steps directly following one another of descaling theroughed out strip, reducing the roughed out strip to final thickness,annealing and pickling.

OBJECT OF THE INVENTION

It is the object of the invention to carry out the pickling without anadditional blast process for descaling and above all to lower the costfor the pickling.

SUMMARY OF THE INVENTION

The object set forth is achieved according to the invention in that thehigh-quality-steel hot-rolled strip is pickled in the pickling stagewith hydrochloric acid and in a first rinsing stage is washed withnitric acid or an equivalent replacement pickling medium and in furtherrinsing stages is treated with rinse water. The method thus is carriedout without an additional blast operation and the costs for the acidsare reduced. In pickling with nitric acid or an equivalent replacementpickling medium, still remaining scaling residues on the strip areremoved. The use of a blast chamber is not required. Since the annealingof the high-grade-steel hot-rolled strip, depending upon the furtherprocessing is not mandatory, the costly production with an annealing andpickling line under earlier circumstances can be bypassed. Theprepickling in hydrochloric acid reduces the amount of metal dissolvedin the nitric acid drastically. Thus there is a reduction of knownproblematical reaction products (No_(x)-gas in the effluent air andnitrate in the effluent water associated with the method). The knowndrawbacks also can be eliminated by the use of a correspondingreplacement medium for the nitric acid. According to the teachings ofthe invention, in addition, pickled high-quality-steel hot-rolled stripis produced with an improved surface quality. In addition the operatingcosts are substantially lower by comparison to the known method.

In the development of the invention the high quality steel hot rolledstrip is pickled in at least one pickling stage with hydrochloric acid,the advantage is the drastic reduction in the dissolved metal quantityby prepickling.

A further improvement resides in that the high quality steel hot rolledstrip is treated in at least one or in a number of rinsing stages withthe nitric acid or an equivalent replacement medium. Advantageously acomplete descaling is achieved.

According to a further feature it is provided that the high-qualityhot-rolled steel strip is then treated in two or more heating stageswith rinse water. That eliminates completely the acids used.

The improvement of the invention resides further in that as theequivalent replacement pickling medium a pickling medium containinghydrogen peroxide is provided. This pickling medium operates in the samemanner and way as the nitric acid.

The equivalent replacement pickling medium can be formed by a mixture ofsulfuric or hydrochloric acid with hydrogen peroxide and otheradditives.

The apparatus for pickling high-quality-steel hot-rolled strip with anaustenitic, ferritic and/or martensitic structure which can be fedthrough a pickling line optionally downstream of a soft annealingfurnace achieves the objects set forth according to the invention inthat the high-quality hot-rolled strip is treatable in a pickling linewhich is comprised of a plurality of pickling tanks with hydrochloricacid and a plurality of rinsing tanks with nitric acid or a replacedpickling medium and further rinsing tanks with rinse water. Theadvantages are those already given for the improvements of the methodsteps over the state of the art.

The number of pickling stages can be varied. A preferred arrangement isto provide in the pickling line that the first three pickling tanks areto be filled with hydrochloric acid.

The prepickling can also be extended or shortened by varying the numberof pickling stages in that the pickling line the rinsing stages haverinsing tanks with nitric acid or a replacement pickling medium and thenrinsing tanks with rinse water.

An advantageous development is thereby proposed in which the rinsingstages are formed by at least one rinsing tank with nitric acid or witha replacement pickling medium and from at least two rinsing tanks withrinse water.

BRIEF DESCRIPTION OF THE DRAWING

An embodiment of the pickling line is shown in the drawing and themethod is described below in connection therewith. In the drawing:

FIG. 1 is a diagram of a complete strip pickling apparatus;

FIG. 2 is a block diagram for comparison of the pickling stage of astandard steel production;

FIG. 3 is a block diagram of the pickling stages of the pickling andrinse tanks in high quality steel production.

SPECIFIC DESCRIPTION

According to FIG. 1, the high-quality steel e.g. stainless steelhot-rolled strip 1 from strip coils of an inlet section 2 is passedthrough a welding device 3 (for a coil replacement) and through atrimming machine 4 into an inlet loop former 5, and if required througha soft annealing furnace 6 into the pickling line 7. The pickling line 7comprises pickling stages 8 and rinsing stages 9, which will bedescribed in greater detail hereinafter. From the last rinsing stage 9,the high quality hot strip 1 passes through an outlet loop former 10 andan outlet shear 11. During a continuous operation the high quality steelhot strip 1 is wound up again in coils in the outlet section 12.

According to FIG. 2 during normal steel production the pickling line 7in the pickling stage 8 is operated with three pickling tanks andhydrochloric acid while the rinsing stage operates totally with rinsewater (rinses 1 and 2, rinses 3-6).

According to FIG. 3, the pickling line 7 is constructed as follows. Inpickling tanks 16, the pickling line 7 has in the pickling stages 8 asthe pickling medium 14 a, hydrochloric acid 13. The rinsing stages 9 areformed by rinsing tanks 17 in which rinse Nr. 1 and rinse Nr. 2 containnitric acid 14 or a replacement pickling medium 14 b. In the followingrinsing stages 9, rinse water 15 is used.

The method is carried out so that the high quality steel hot strip ispickled in the pickling stages 8 with hydrochloric acid 13 and in thefirst rinsing stage 9 with nitric acid 14 or a equivalent replacementpickling medium and in the further rinsing stage with rinse water 15.Two or more pickling stages 8 with hydrochloric acid can be used. Alsothe high quality steel hot strip 1 can be treated in two or more of therinsing stages 9 with the nitric acid 14 or an equivalent replacementpickling medium 14 b. Then the high quality hot steel strip 1 can berinsed in two or more rinsing stages.

1. A method of pickling high-quality steel strip consisting essentiallythe steps of: (a) passing high-quality hot-rolled steel strip with anaustenitic, ferritic or martensitic structure through a soft annealingfurnace immediately followed by a pickling line consisting essentiallyonly of pickling and rinsing stages; (b) in at least one pickling stageof said pickling stages, initially pickling said high-quality hot-rolledsteel strip with hydrochloric acid; (c) in at least one rinsing stage ofsaid rinsing stages following step (b), rinsing said high-qualityhot-rolled steel strip with nitric acid or a mixture of sulfuric acidwith hydrogen peroxide or a mixture of hydrochloric acid with hydrogenperoxide; and (d) thereafter rinsing said high-quality hot-rolled steelstrip with water in at least one further one of said rinsing stages. 2.The method defined in claim 1 wherein said high-quality hot-rolled steelstrip is pickled in step (b) in at least two of said pickling stageswith hydrochloric acid.
 3. The method defined in claim 2 wherein saidhigh-quality hot-rolled steel strip is rinsed in step (c) in at leasttwo of said rinsing stages with nitric acid or a mixture of sulfuricacid with hydrogen peroxide or a mixture of hydrochloric acid withhydrogen peroxide.
 4. The method defined in claim 3 wherein saidhigh-quality hot-rolled steel strip is rinsed in step (d) with water inat least two of said rinsing stages.
 5. The method defined in claim 1,wherein said high-quality hot-rolled steel strip is rinsed in step (c)in at least two of said rinsing stages with nitric acid or a mixture ofsulfuric acid with hydrogen peroxide or a mixture of hydrochloric acidwith hydrogen peroxide.
 6. The method defined in claim 1 wherein saidhigh-quality hot-rolled steel strip is rinsed in step (d) with water inat least two of said rinsing stages.
 7. An apparatus for picklinghigh-quality steel strip comprising: a soft annealing furnace; and apickling line positioned immediately downstream of said furnace andtraversed by high-quality hot-rolled steel strip with an austenitic,ferritic or martensitic structure, said pickling line consistingessentially only of pickling and rinsing stages and including: at leastone pickling tank of said pickling stages for initially pickling saidhigh-quality hot-rolled steel strip with hydrochloric acid, at least onerinsing tank of said rinsing stages following pickling with hydrochloricacid for rinsing said high-quality hot-rolled steel strip with nitricacid or a mixture of sulfuric acid with hydrogen peroxide or a mixtureof hydrochloric acid with hydrogen peroxide, and at least one furtherrinsing tank of said rinsing stages for thereafter rinsing saidhigh-quality hot-rolled steel strip with water.
 8. The apparatus definedin claim 7 wherein said pickling stages have three pickling tanks withhydrochloric acid traversed in succession by said high-qualityhot-rolled steel strip.
 9. The apparatus defined in claim 8 wherein saidrinsing stages have two rinsing tanks with nitric acid or a mixture ofsulfuric acid with hydrogen peroxide or a mixture of hydrochloric acidwith hydrogen peroxide following the three pickling tanks withhydrochloric acid.
 10. The apparatus defined in claim 9 wherein saidrinsing stages have a plurality of tanks with water following the tworinsing tanks with nitric acid or a mixture of sulfuric acid withhydrogen peroxide or a mixture of hydrochloric acid with hydrogenperoxide.